Some observers interpreted the move, which followed months of delays in Flynn’s sentencing, as an indication that Mueller is unlikely to call Flynn to testify at any future trials that may arise from the probe into Russian interference in the 2016 presidential election.

“It’s a sign that, perhaps, Flynn may not be critical to other pieces of Mueller’s investigation,” said Ron Hosko, a former assistant director at the FBI.

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But Hosko and others warned that while Mueller may be wrapping up with Flynn, the development should not be construed as a sign that Mueller is concluding his broader probe, which includes examining possible collusion between Trump’s campaign and Moscow and whether the president obstructed justice.

“These are the types of moves a prosecutor makes when they are entering the final stage of an investigation or prosecution,” said Jonathan Turley, a law professor at George Washington University. “What people have to be cautious about is that Mueller could be wrapping up aspects of his investigation while continuing to explore other aspects.”

“The Mueller investigation is now so sprawling that it is dangerous to assume that shutting down one aspect will necessarily mean the end to all aspects of his investigation,” said Turley, who is also an opinion contributor for The Hill.

Flynn pleaded guilty in December to one count of lying to FBI agents about his contacts with the Russian ambassador to the United States and agreed to cooperate with Mueller’s team, from submitting to interviews with government investigators to providing courthouse testimony.

From the start, Flynn has been viewed as a valuable witness in Mueller’s investigation, given his prominent role on the campaign and brief tenure as national security adviser. Court filings show that Flynn communicated with senior members of Trump’s transition team about his conversations with then-Russian Ambassador Sergey Kislyak about U.S. sanctions on Moscow.

Mueller had sought to delay Flynn’s sentencing four times, a sign that as of late August his cooperation was still needed months after his guilty plea. Early on, there was speculation in conservative circles that Flynn’s plea deal could be collapsing, with some alleging that Flynn had been set up by the FBI.

In a joint filing with Flynn’s defense attorneys on Monday, Mueller’s team requested that Judge Emmet D. Sullivan set Flynn’s sentencing date for Nov. 28 or thereafter, a sign that Mueller has gotten all the information he needs from Flynn for the purposes of the investigation into Russian interference.

But some legal analysts downplayed Monday’s action, noting that Mueller could still call Flynn to testify after he is sentenced.

“It’s an insignificant development on the testimony front. I think if Mueller intended to call him, he’s going to call him to testify,” said Glenn Kirschner, a former federal prosecutor in D.C. “The one thing that it convinces me of is Flynn’s investigative usefulness is over.”

Prosecutors often will delay a defendant’s sentencing until after their testimony in any pertinent cases, a move that provides leverage over the defendant and incentivizes them to testify to the best of their ability.

But because Flynn is likely to face a minimal sentence for his crime, some argue that it is unlikely that any testimony he makes in court would have a major impact on his sentencing.

“Flynn probably isn’t going to go to jail,” said Randall Eliason, a former assistant U.S. attorney in D.C. “There’s not much reason to delay it, from either side’s point of view.”

Mueller’s request is likely to set up the former Trump adviser, who has made few public appearances since his guilty plea, for a key court appearance just after the November midterm elections. It is possible that Flynn’s sentencing could shed more light on his cooperation with the government, though Mueller’s team could file documents under seal so as to shield the public from sensitive information about the investigation.

For the time being, the extent and nature of Flynn’s cooperation remains shrouded in mystery.

Meanwhile, Trump and his allies are anxious for the probe to wrap up, and the developments with regard to Flynn are unlikely to alleviate frustrations at the White House.

“On one side for the White House, I think it’s a recognition that Mueller and the special counsel’s office are done with Flynn and likely the collusion question,” said GOP strategist Ford O’Connell. “But the White House is still fuming because what you have here is an investigation in search of a crime.”

The White House did not respond to a request for comment.

Flynn’s crime carries a sentence of up to five years in prison and a fine of as much as $250,000, though the federal sentencing guidelines call for a prison term of between zero and six months and a fine of between $500 and $9,500.

Many observers view the developments in Flynn’s case as a step forward in an investigation that has lasted 16 months so far. Still, Mueller has revealed little about his game plan beyond what is included in indictments and said in court appearances, offering few clues of where the broader investigation stands.

“There’s an urgency to everything he’s doing,” said Hosko, who worked under Mueller when he was FBI director. “He’s trying to get to the core truth of what his tasking has been, and I don’t think speed is of the essence to him, though I believe it’s fully on his mind.”